It’s a story that potentially explains why Earth hasn’t yet been contacted by alien life. There might be a very difficult obstacle every intelligent life form eventually faces, an obstacle greater than all the ones before it, that almost no species surmounts. This obstacle permanently extinguishes (filters) almost all life forms. If the Great Filter exists, it’s an evolutionary challenge humans could face with extremely low probability of success.

There’s an excellent article discussing The Great Filter (and other responses to Fermi’s Paradox) on the blog Wait But Why. And the authors note that, if the Great Filter exists, we don’t know whether we’ve already encountered it or not. WBW writes:

Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there are thousands of them just in our own galaxy, something else must be going on.

This something else is called The Great Filter.

The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.

For example, regardless of what one thinks of the possibility of anthropogenic climate change in the short term (the next 200 years), the fact is that Earth’s climate WILL change in the long term (in the next 200,000 years). Drastically. You could imagine how dramatic and geologically quick swings in climate could challenge a species. There are multiple phyla on earth whose species span extreme climates, like cyanobacteria and tardigrades.

How does this relate to moving off-world?

Finding ways to adapt to different living conditions is a useful step toward passing the Great Filter. Let’s diversify our real estate strategy. Hedge our bets.